Catalyst systems which are solutions of one or more catalyst components (e.g. a transition metal compound and optionally a cocatalyst) are known in the filed as homogeneous catalyst systems. Homogeneous systems are used as liquids in the polymerisation process. Such systems have in general a satisfactory catalytic activity, but their problem has been that the polymer thus produced has a poor morphology (e.g. the end polymer is in a form of a fluff having a low bulk density). As a consequence, operation of slurry and gas phase reactors using a homogeneous catalyst system cause problems in practice as i.a. fouling of the reactor can occur.
The above problems have been tried to overcome in several ways: The homogeneous system has been prepolymerised with an olefin monomer before the actual polymerisation step. Said prepolymerisation, however, has not solved the problem of the formation of a polymer fluff. EP 426 646 of Fina has further suggested to use specific prepolymerisation conditions, i.e. the reaction temperature and the reaction time of a narrow, specific range, for improving the morphology of the polymer thus obtained.
In WO 98 37103 the homogeneous catalyst system is introduced as droplets of a certain size into the polymerisation reactor for controlling the average particle size of a polyolefin produced in gas phase polymerisation. Said droplets are formed just before the introduction by using an atomizer (e.g. a spray nozzle).
Furthermore, to overcome the problems of the homogeneous systems in a non-solution process the catalyst components have been supported, e.g. their solution impregnated, on porous organic or inorganic support material, e.g. silica. These supported systems, known as heterogeneous catalyst systems, can additionally be prepolymerised in order to further immobilise and stabilise the catalyst components.
However, also supported and optionally prepolymerised systems present problems. It is difficult to get an even distribution of the catalyst components in the porous carrier material; and leaching of the catalyst components from the support can occur. Such drawbacks lead to an unsatisfactory polymerisation behaviour of the catalyst, and as a result the morphology of the polymer product thus obtained is also poor. Furthermore, the uneven distribution of the catalyst components in the support material can have an adverse influence on the fragmentation of the support material during the polymerisation step.
The support can also have an adverse effect on the activity of the catalyst, on its polymerisation behaviour and on the properties of the end polymer.
Accordingly, various measures have been proposed to improve the morphology properties of homogeneous catalyst systems. However, due to the complexity of the catalyst systems, the need still exists to develop further catalyst systems and preparation methods thereof which overcome the problems of the prior art practice.